Dissociative Ionization Mechanism and Appearance Energies in Adipic Acid Revealed by Imaging Photoelectron Photoion Coincidence, Selective Deuteration, and Calculations

Abstract

Adipic acid, a model compound for oxygenated organic aerosol, has been studied at the VUV beamline of the Swiss Light Source. Internal energy selected cations were prepared by threshold photoionization using vacuum ultraviolet synchrotron radiation and imaging photoelectron photoion coincidence spectroscopy (iPEPICO). The threshold photoelectron spectrum yields a vertical ionization energy (IE) of 10.5 eV, significantly above the calculated adiabatic IE of 8.6 eV. The cationic minimum is accessible after vertical ionization by H-transfer from one of the γ-carbons to a carbonyl oxygen and is sufficiently energetic to decay by water loss at the ionization onset. The slope of the breakdown curves, quantum chemical calculations, and selective deuteration of the carboxylic hydrogens establish the dissociative photoionization mechanism. After ionization, one γ-methylene hydrogen and the two carboxylic hydrogens are randomized prior to H2O loss. On the basis of the deuteration degree in the H2O + CO-loss product at higher energies, a direct water-loss channel without complete randomization also exists. The breakdown diagram and center of gravity of the H2O + CO-loss peak were modeled to obtain 0 K appearance energies of 10.77, 10.32, and 11.53 eV for H2O + CO loss, CH2COOH loss, and H2O + CH2COOH loss from adipic acid. These agree well with the CBS-QB3 calculated values of 10.68, 10.45, and 11.57 eV, respectively, which shows that threshold photoionization can yield energetics data as long as the dissociation is statistical, even when the parent ion cannot be observed. The results can be used as a starting point for a deeper understanding of the ionization and low-energy fragmentation of organic aerosol components.

Valence Photoionization of Thymine: Ionization Energies, Vibrational Structure, and Fragmentation Pathways from the Slow to the Ultrafast

“…This result emphasizes the inherentd ifficulty in interpreting photoelectron spectra without FCF calculations:N ot only may vibrationalt ransitions be misidentified, but the nature of the electronic states too, [50] and even the adiabatic ionization energies may lie significantly below or above the ionization onset, depending on the geometry change upon ionization andt he source temperature. [51][52][53]…”

The Distant Double Bond Determines the Fate of the Carboxylic Group in the Dissociative Photoionization of Oleic Acid

“…In large molecules, even minor conformational change may suppress the nuclear wave function overlap for the origin transition, and shift the ionization onset to higher energies than the adiabatic ionization potential. These DFT results suggest that the ionization energy could lie slightly below the extrapolated onset of the TPES, but less so than in adipic acid, in which the difference was found to be 1.5 eV . The minimum in the TPES around 9.6 eV reproduces the literature spectrum well, after which the signal continues to rise, in line with the increased density of molecular orbitals.…”

“…These DFT results suggestt hat the ionization energy could lie slightly below the extrapolatedo nset of the TPES, but less so than in adipic acid, in whicht he difference was found to be 1.5 eV. [24] The minimum in the TPES around9 .6 eV reproduces the literature spectrum well, after which the signal continues to rise, in line with the increased density of molecular orbitals. The HOMO and HOMO-1 DFT orbital energies at À6.68 eV and À7.92 eV are followed by MOs at energies of À8.36, À8.63, À8.80, À8.83 eV etc.,w hich, when offset to match the ionization energya ta round8 .5 eV, [60] confirm the large number of electronically excited states above 10 eV.B ecause of sample inlet instabilities, the TPES could not be plotted above 10.2 eV herein.…”

“…In adipic acid cations, a g-( or b-, depending on the reference carboxylic group) hydrogen atom was found to transfer to the carboxylic groups irreversibly and withouta no verall barrier in the first step of the McLafferty rearrangement, but insteado fb-cleavage afterwards, it eventually movest oah ydroxyl group,f orming al eaving water molecule. [24] However, the adipic acid mechanism cannotb ec arried over to oleic acidb ecause of three reasons: 1) in the absence of as econd hydroxyl group, the first step of McLafferty rearrangementc ould not be followed by the formation of aw ater leaving group, 2) this mechanism does not dependonw hether the carbon chain is unsaturated, and 3) water loss is followed by CO-loss in adipic acid, whereas sequential CO-loss is not observedi no leic acid. Yet, the dominance of water loss at lower energies strongly suggests that it has au nique reaction coordinate, distinct from the manifold of higher energy fragmentation channels, and its identification maya lso shine light on the role of the C=Cd ouble bond in water loss.…”

“…Furthermore, recent studies on medium-sized organics have shown that internal energy selection in threshold photoionization is ap owerful tool to determine complicated fragmentation mechanisms unambiguously when combined with ab initio calculations. [24,35] The breakdown diagram and TOF distributions give clues to the barrierh eights and can be used to dis-tinguishp arallel and sequential processes,a sw ell. Combined with reactionp ath calculations that yield consistente nergetics, complex dissociation reaction paths can, thus, be readily unveiled.…”